A switched mode power supply may include a switching transistor which is coupled to the primary winding of a transformer and to a DC input voltage derived from the AC mains supply. The switching transistor is periodically switched between conduction and cutoff by a control voltage developed by a control circuit. The control voltage may be pulse width modulated, the modulation being a function of the amplitude of the generated operating voltage at the secondary side of the transformer. The modulation is such that the conduction interval or on-time of the switching transistor is modified so as to stabilize the output voltage.
During the on-time of the switching transistor, magnetic energy is stored in the transformer from the input voltage in the form of an upramping current flowing in the primary winding of the transformer. After the switching transistor is cutoff, this stored magnetic energy is transferred via induced voltage pulses to the secondary windings of the transformer. A corresponding charging current passes into the filters of the secondary side rectifier circuits.
The energy stored in the transformer is discharged into the secondary side rectifier circuits during the off-time of the switching transistor. The off-time is terminated when the switching transistor is again switched into conduction and the transformer is again charged with magnetic energy in the form of current flowing in the primary winding.
In one form of switched mode power supply, the stored magnetic energy is discharged into the secondary side rectifier circuits during the off-time of the switching transistor until current in the transformer secondary windings decreases to zero. Thereafter, a deadtime interval commences when the switching transistor and the rectifier circuits are nonconductive. The deadtime interval is terminated when the switching transistor is again switched into the conductive state.
During a startup interval, transient startup effects within the switched mode power supply may adversely affect the ability of the control circuit to generate a control voltage that is adequate to safely produce on-off switching of the output switching transistor. For example, the control circuit may be capable of building up the control voltage to a level which can turn on the switching transistor, but which cannot adequately turn off the transistor after it has been turned on. In particular, the control circuit may undesirably permit excessively long duration on-time intervals for the switching transistor. An excessively long duration on-time permits the upramping current in the transformer primary winding to increase to an excessively large amplitude before the transistor is switched off. As a result, excessively large voltage and current pulses may be generated in the transformer windings. These pulses may be severe enough to damage components in the switched mode power supply and the loads circuits coupled thereto.
A feature of the invention is a switched mode power supply which safely makes the transition out of the startup mode of operation.